It is known to supply heat to boilers for heating hot service water, for example by means of heat pumps. The conveying of the heat from the heat pump into the boiler occurs through a cooling medium or refrigerant, which can for example be obtained in commerce under the names FRIGEN or GLYKOL. Since these cooling mediums are highly poisonous, measures must be taken in order to be able to prevent with certainty any part of the cooling medium from leaking into the water which is provided in the boiler. For this reason, the heating coil which is connected to the heat pump is constructed as a double sleeve pipe, the outer side of the inner pipe having webs which engage the inner wall of the outer pipe. The remaining space between the outer and inner pipes is filled with air. Through this construction, however, only limited heat transfer can be achieved, and such heat exchangers thus have a very poor efficiency.